1. Field of the Invention
This invention pertains to the matrix rate treatment of wells and more in particular to the sequential treatment of formation strata by the temporary closing of perforations in the well casing during the treatment.
2. Description of the Prior Art
It is common practice in completing oil and gas wells to set a string of pipe, known as casing, in the well and use cement around the outside of the casing to isolate the various formations penetrated by the well. To establish fluid communication between the hydrocarbon bearing formations and the interior of the casing, the casing and cement sheath are perforated.
At various times during the life of the well, it may be desirable to increase the production rate of hydrocarbons through a matrix treatment stimulation. Matrix treatments are stimulation treatments which are injected at pressures below the fracture pressure of the formation. In other words, the fluid is being forced into the formation at a rate such that the pores of the formation accept the flow without fracturing the formation. A common example of a matrix rate treatment is matrix acidization whereby an acid bearing fluid is injected into the formation so that the acid can permeate into the near wellbore area of the formation and increase permeability. Generally, acidization is limited to within a few feet of the wellbore. The purpose of a matrix acidization treatment is to dissolve near wellbore damage such as clays and formation fines which clog or constrict the formations' fissures and channels. Other types of well treatment fluids such as solvent surfactants can also be applied in matrix rate treatments.
It is the objective in a matrix treatment stimulation to inject the treating fluid into the zones of the formation where treatment is required. But as the length of the perforated pay zone or the number of perforated pay zones increases, the placement of the fluid treatment in the regions of the pay zones where it is required becomes more difficult due to differences in formation characteristics. For instance, the strata having the highest permeability will most likely consume the major portion of a given stimulation treatment leaving the least permeable strata virtually untreated. Therefore, techniques have been developed to divert the treating fluid from its path of least resistance so that the low permeability zones are also treated.
One technique for achieving diversion involves the use of particulates such as rock salt and benzoic acid flakes. Typically particulates are solids having limited solubility in the treating fluid but are soluble in the produced fluids. The particulates are added to the treating fluid during the treatment and plug the formation as they are carried by the fluid through the perforations and into the formation pores. As certain sections of the formation get plugged, the treating fluid is diverted and forced to flow into the unplugged sections of the formation. The formation is unplugged after the treatment by dissolving the particulates as the well is either flushed or produced. Dissolving the particulates can at times be a difficult task. The major drawback of particulates is that if the proper fluid which will dissolve the particulates cannot be brought into contact with the particulates, the particulates will remain solid particles blocking the flowpaths for the produced fluids into the well, thereby permanently damaging the production capability of the well and defeating the purpose of the treatment.
Ball sealers provide a diverting technique which avoids this problem. Ball sealers are small rubber coated balls which are sized to seal off the perforations inside the casing. When ball sealers are used, they are pumped into the wellbore along with the treating fluid. The balls are carried down the wellbore and onto the perforations by the directional flow of the fluid through the perforations into the formation. The balls seat upon the perforations and are held there by the pressure differential across the perforations. The major advantages of utilizing ball sealers as a diverting agent are their ease of use, positive shut-off, independence of formation conditions, and inertness. The ball sealers are simply injected at the surface and transported by the treating fluid to the perforations to be plugged. Other than a ball injector, no special or additional treating equipment is required. The ball sealers are designed to have an outer covering sufficiently compliant to seal a jet or bullet formed perforation and to have a solid, rigid core which resists extrusion into or through the perforation. Therefore, the ball sealers will not penetrate the formation and permanently damage the flow characteristics of the well.
Although ball sealers have been frequently and successfully used as diverting agents in fracturing operations, they have rarely been used as diverting agents in matrix rate treatments because in matrix treatments they generally have been ineffective. Their ineffectiveness is due to the relatively low flow rate of the treating fluid through the perforations during a matrix treatment. The seating efficiency of most commercially available ball sealers used according to present-day practices is a function of the flow rate through the perforations. It has been generally accepted in the art that the greater the flow rate of the treating fluid through the perforations, the greater the seating efficiency of the ball sealers will be. When the flow rate through the perforations is very low, the seating efficiency of ball sealers, as presently used, is extremely low because the low flow rate will not effectively carry the ball sealers to the perforations before they sink past the perforations. Since ball sealers have been so ineffective in diverting matrix rate treatments, they have rarely been used in such treatments.